30
4929B–AUTO–01/07
ATA6264 [Preliminary]
10. VSAT Power Supply
A stabilized VSAT supply is realized by a buck converter. An external inductance is
PWM-switched with a frequency of 200 kHz via an internal high-side DMOS power transistor.
The VSAT power supply is connected to the boost converter output (EVZ), and uses the stored
energy of the boost converter capacitor if the voltage at K30 is missing. The regulator uses both
current and voltage feedback. The basis for the regulation loop is a temperature-compensated
band-gap reference voltage, which is compared with the internally divided output voltage VSAT.
The error amplifier output is applied to the inverting input of a comparator, the current feedback
is connected with the positive input. The PWM flip-flop (which is set every 5 µs by the oscillator)
is reset if the current feedback reaches the error amplifier level. In order to adjust the compensa-
tion of the regulation loop and therefore improve the behavior in case of load changes in
continuous-mode operation, pin COMSATO has to be connected to COMSATI via a compensa-
tion network. Because of the fact that current-mode-controlled converters exhibit sub-harmonic
oscillations when operating at duty cycles higher than 50%, a slope compensation (which adds
an artificial ramp to the comparator) is implemented. If the regulator input voltage at pin EVZ is
too low, the regulator switches to a duty cycle of 100% (Permanent-on mode).
The VSAT voltage can be programmed via the serial interface to one of three different voltage
values during initial programming.
Figure 10-1. Functional Principle of the VSAT Regulator
The duration of the output transistor conduction depends on the VSAT level and current feed-
back. Conduction is suppressed immediately if the current through the output transistor exceeds
850 mA typically. A logic circuit disables, in the case of short spikes, multiple-pulse operation
during one oscillating period. If pin VSAT is open (VSAT loss), an internal current source con-
nected to a higher voltage than VSAT acts as pull-up for this pin, to prevent the VSAT voltage
from rising up to EVZ. In order to ensure the gate voltage for the output transistor, the driver
stage is supplied by the charge pump (pin CP).
VSAT
VSAT
-
+-
+
SVSAT
EVZ
Logic and
driver
Slope
compensation
Comp. OSC
Error
amp.
Bandgap
reference
Overvoltage
Overcurrent
Q
R
S
Current
measurement
and leading edge
blanking
SPI
OTP
+
COMSATI
COMSATO
CP